ecs-metadata: support dimensionToUpdate config field

[rmfitzpatrick]

These changes add a dimensionToUpdate config field to the ecs-metadata monitor to allow configuring dimension updates on either the container_name and container_id dimension. Since container_name's can be long lived, perpetuating entities to update becomes unnecessary and setting to the more ephemeral container_id is preferable. My plan is to update the default value to container_id in a subsequent release but can understand wanting to here instead.

edit: updated to sync on container_id by default, w/ bw-compatibility option w/ new config field.

[rmfitzpatrick]

@dloucasfx would appreciate your thoughts on if this is too conservative and we should simply update the dimension to sync to container_id without a transitional option and future default value change.

edit: I've gone ahead and updated to container_id without an intermediary default change w/ a breaking change note.